Is there any way to mirror only selected parts of a shape key?
Other answers so far are good, but it's worth pointing out another technique.
There's an easy way to restore part of a shapekey to the basis shape, using &quot;blend from shape&quot; operation.  You can use this on parts of the mesh to &quot;unshape&quot; bits of the mesh.
Select the shapekey, then enter edit on the mesh.  Select all vertices of the shapekey you don't want (or select the bits you do want, then select inverse.)  Use a &quot;blend from shape&quot; operation, which I always call from the searchbar.  On the operator panel, specify Basis as the shapekey, set the blend to 1.0, and disable Add.  This restores the selected vertices to the Basis shape, making it so that some of your shapekey will be &quot;unmirrored&quot;.
You can of course use this operator for a lot of other things as well.